A survey of hemoglobinopathies which was carried out in the Takamatsu district during the period from January to August 1979 detected six families with abnormal hemoglobins. Approximately 6010 inhabitants were screened. Three of these families had the same new Hb variant (Hb Takamatsu beta 120 Lys leads to Gln) that has not been previously reported. Existence of a blood relationship among these three families could not be established even after careful family studies. This abnormal hemoglobin was not associated with adverse symptoms and gave normal hematologic findings in the carriers. The isopropranol test was negative, oxygen affinity was within the normal range, and biosynthetic ratio in reticulocytes was around 1.0. One of the difficulties in the structural analysis of this hemoglobin was related to complete superposition of abnormal beta XT-12b,13 on a beta T-8,9 peptide in the fingerprint of the trypsin digest of aminoethylated aberrant beta X chain. This was overcome by collection of abnormal tryptic beta core (beta XT-10-13) from unmodified beta X chain, and subsequent digestion by chymotrypsin. Edman analysis of the chymotryptic peptides thus obtained successfully confirmed the substitution to be beta 120 Lys leads to Gln.